1. Field of the Invention
This invention relates to an improvement in a conventional method for producing a pressed glass article, and more particularly relates to the cooling and lubrication of conventional glass pressing apparatus which can be used to produce such an article.
Pressed glass articles, e.g., table ware, lenses, insulators, glass blocks, white ware and crystal ware, are usually made by placing a charge of heat-softened glass, commonly known as a "gob", in a cavity of a mold, and then advancing a plunger, under pressure, into the mold. Modern manufacturing processes, which generally enable large quantities of pressed glass articles of precise dimensions to be produced in a relatively short time, usually employ a plurality of molds. The cavities of the molds are charged with gobs of softened glass from a central reservoir, and the gobs are formed into pressed articles by advancing into the molds plungers driven, for example, hydraulically or by air. Typically, automated pressing operations are used; in such operations several molds are mounted on a circular table and rotated step by step to bring each mold, in turn, below a charging device, then to a pressing station and, after a sufficient interval for cooling, to a station where the pressed articles are removed from the molds.
Particularly in automated glass pressing operations, a major consideration is the control of heat distribution and removal so that surfaces of the pressing apparatus which form the shape of the glass articles, e.g., surfaces of the plunger and the mold cavity, do not become either too hot or too cold. If these surfaces become too hot, sticking of the glass to the mold or plunger may occur; if they become too cold, a crizzling or checking of the glass can result. Overheating can also be detrimental to the proper functioning and longevity of the pressing apparatus, and especially detrimental to metal working surfaces thereof. Expansion of such surfaces can cause, for example, excessive friction and wear as the apparatus is operated. Consequently, during a pressing operation the rate at which excess heat is removed from the pressing apparatus usually limits the rate at which pressed glass articles can be produced from that apparatus.* FNT *The repeated exposure of the pressing apparatus to molten glass during a pressing operation causes temperatures of parts thereof to rise rapidly; it is thus desirable that the parts be cooled to within acceptable working ranges. If the mold, for examle, is not cooled sufficiently, especially in those localized portions which contact molten glass, it will reach a temperature at which the glass will begin to stick to its interior forming surface, a condition which can necessitate stoppage of the pressing operation for repair of damage. If the temperature of the mold or plunger is not maintained, for example, or is permitted to fall below a predetermined level, wrinkles, checks and other imperfections can result in the finished article. Localized cooling of glass forming surfaces is consequently desirable; it is evident that there is a desirable working temperature range within which the various parts of the pressing apparatus should be maintained, and that the amount of heat which must be removed depends on such factors as, for example, the speed of the pressing operation and the temperature of the molten glass charged to the mold.
It has been found desirable not only to cool surfaces of glass pressing apparatus to prevent overheating, but also to lubricate, for example, the plunger, which contacts the molten glass as it is advanced into the mold to press the article. Proper lubrication helps to prevent adhesion of the glass article thereto when the plunger is subsequently removed from the mold, and can also help to minimize wear of the plunger and other parts of the pressing apparatus.
2. Description of the Prior Art
Various methods have heretofore been used to cool glass pressing apparatus and to lubricate surfaces thereof. A common practice is to remove heat from such apparatus by forcing compressed air, water, natural gas or the like through passages in the mold and/or in a manifold surrounding the exterior of the plunger. However, the use of compressed air for cooling is expensive and also limits production rates because of its low cooling capacity. When water cooling systems are used, including both closed systems, and open, spray-type systems, there is a tendency for an insulating film to form, limiting cooling; an open system causes steam, which is detrimental to both the apparatus and its operator. Furthermore, whether a water system is open or closed, collection of the used cooling water is usually necessary, often resulting in additional expense. A liquefied petroleum gas, for example "natural gas", while ordinarily a satisfactory coolant, and easily disposed of by ignition following use, offers certain disadvantages, as described below.
Lubricants for parts of glass pressing apparatus which contact molten glass or other parts of the apparatus are well known. For example, one practice is to coat a plunger of such apparatus by causing kerosene, light oils, or the like to flow from a manifold or other apparatus around its exterior to deposit a lubricious coating on the plunger during a pressing operation. Lubricants, for example, mixtures of kerosene and light oil, are particularly effective when ignited by contact with molten glass in the mold or by an external spark or pilot light.
While compressed air, water and liquefied petroleum gases can assist in cooling, these substances are not satisfactory for the lubrication of glass pressing apparatus. Desirably, a single application should provide lubrication and remove heat; kerosene, light oil, unsaturated petroleum gases and mixtures of such gases are comparatively undesirable because their ignition often produces unacceptable levels of carbonaceous residue on the mold and glass article, in addition to atmospheric pollution. Liquefied alkanes can be used, and, when ignited, generate less residue and pollution than the aforementioned substances, but do not afford adequate lubrication. The inadequacy of alkanes as lubricants is believed to be attributable to their low carbon contents.